What do I need to turn off an LED 10 seconds after it powers on?

[Mashoko]

browse for a "single transistor monostable".

 

[colin55]

Why use an expensive MOSFET when a 3 cent transistor will do the same thing and the circuit will work on 3v for a red LED.

 

[mbarazeen]

alos in BJT design, the standby current is very less since transistor would remain off & no base current.leakage through the capacitor can be same as MOSFET. but do some experiment changing resistor values(or calculate) and find the optimum capacitor size for both cases for particular voltage of choice. the smaller the capacitor the lesser the energy stored and lost each time.

 

[Boncuk]

Hi GT3,

concerning the pin count and number of parts (hence overall size of the circuit) I guess the best solution is still a transistor.

Using an integrated circuit you still require the timing components (R and C) + a current limiting resistor for the LED.

You should try to get a 2N6660 (TO39 package) or a BS170 (TO92 package). Both are about the same size (2N6660 has a metal can, and BS170 has plastic)

I reworked the design for an ON-time of 25 seconds and omitted the gate resistor (former R1). The cap has been changed to 10µF and the charge resistor to 3.3MΩ.

The LED stays on brightly for 25 seconds and fades out for further 5 seconds.

A green LED works fine, but you must observe to use a low current LED (If=2mA instead of a normal LED with an If of 20mA) as specified in the circuit.

The final current flow of 3µA is not affected by changing the values of the timing components.

Regards

Boncuk

 

[pspec]

Hi GT3,

concerning the pin count and number of parts (hence overall size of the circuit) I guess the best solution is still a transistor.

Using an integrated circuit you still require the timing components (R and C) + a current limiting resistor for the LED.

You should try to get a 2N6660 (TO39 package) or a BS170 (TO92 package). Both are about the same size (2N6660 has a metal can, and BS170 has plastic)

I reworked the design for an ON-time of 25 seconds and omitted the gate resistor (former R1). The cap has been changed to 10µF and the charge resistor to 3.3MΩ.

The LED stays on brightly for 25 seconds and fades out for further 5 seconds.

A green LED works fine, but you must observe to use a low current LED (If=2mA instead of a normal LED with an If of 20mA) as specified in the circuit.

The final current flow of 3µA is not affected by changing the values of the timing components.

Regards

Boncuk

Hi

Sorry for posting on this old topic but I was trying to run this circuit on circuit wizard to see if I could get it to work on that before trying it in the real world.

I have a question with regards the R3 in this diagram it states its value at 150R. in the circuit wizard it does not have an R it has M,K,blank,m,u,n,p, (I put u as that’s the closest letter that looks like the symbol). Is one of these letters the equivalent of R…

I have this and it’s not working

Anyone have any ideas

 

[ericgibbs]

hi .
The 150R means its only 150 Ohms.
M=meg ohms
K=kilo ohm
Blank or R, means Ohms

Use a 150 Ohm resistor.

E.

 

[pspec]

thanks that did it.........